1. Field of the Invention
The present invention relates to apparatus and method for electrostatically depositing coatings. More particularly, the present invention relates to apparatus and method for electrostatically coating substrates, such as sheets or coils of metallic or non-metallic materials, with particulate materials, and for minimizing ecological contamination by depositing smaller particles first.
2. Description of the Related Art
The process of electrostatic depositing is used for depositing various kinds of materials onto metal objects or sheets. Uses for electrostatic depositing include depositing of: paint, dry powder coatings, abrasives, flocking materials, and lubricants. In addition, electrostatic depositing is used to reproduce printed material and pictures by the process that is known as xerography.
Examples of the related art in depositing of lubricants are: Scholes et al., U.S. Pat. No. 4,066,803, issued 3 Jan. 1978; and Jenkins et al., U.S. Pat. No. 2,608,176, issued 16 Mar. 1948. In like manner, Escallon, U.S. Pat. No. 4,526,804, issued 2 Jul. 1985, and Rocks et al., U.S. Pat. No. 3,155,545, issued 27 Feb. 1961, are examples of the related art in depositing granular materials; whereas Wiggins, U.S. Pat. No. 3,937,180, issued 10 Feb. 1976, and Cosentino et al., U.S. Pat. No. 4,742,154, issued 9 Feb. 1988, are examples of patents which teach electrostatic depositing of paint.
Two problems have attended electrostatic depositing. One is that the process of electrostatic depositing can develop a residual electrostatic potential on the coated material. Where materials with dielectric properties, such as lubricants, are deposited, the deposited material can retain a residual electrostatic charge. In the case of electrostatically lubricated metallic sheets, the residual electrostatic charge has caused sheets in a stack to stick together, and has electrostatically attracted contaminants from the air to lodge on the coated material.
The second problem is that of meeting increasingly strict ecological standards in that some of the coating material drifts out, or is blown out, of the depositing chamber.
A primary cause of the coating material drifting out of the depositing chamber is that, as the substrate becomes electrostatically coated, it can acquire the charge of the deposited material, reducing the electrical potential between the charged particles which are to be deposited and the substrate, and thereby allowing charged particles to drift out of the depositing chamber rather than being attracted to the depositing surface.
It has been found that, even though a metallic sheet or coil of metallic material is exposed to contact with the transporting apparatus, the surface of the sheet or coil can retain an electrical potential sufficient to spark to a metallic object that is spaced from the coated sheet or coil. This is particularly true of sheets.
The related art includes some attempts to correct the problem of a residual electrostatic charge. For instance, Gibbons et al., U.S. Pat. No. 3,702,258, issued 7 Nov. 1972, teach a method for neutralizing the residual electrostatic charge that remains after treating a web with an alternating current corona field to increase its printability. The apparatus of Gibbons et al. includes a positively energized roller and a negatively energized roller which contact the web, and a pair of electrodes that are spaced apart from respective ones of the rollers on opposite sides of the web from that of the rollers, and that are connected to a potential that is intermediate of the potentials of the two rollers.
Also, in U.S. Pat. No. 4,517,143, issued 14 May 1985, Kisler teaches passing a randomly charged web through two oppositely-charged electrostatic fields to adjust the electrostatic field charge level to a desired, and uniform, level.
With regard to the ecological problem, the usual attempts have involved pulling excess coating material through the depositing chamber with an air evacuating system. Typical of these systems is Rocks et al., U.S. Pat. No. 3,155,545.